Conventional (ethanol-free) mogas (gasoline sold at the pump for road use) has been largely replaced by ethanol-containing gasoline in the United States; other countries are also mandating the use of oxygenates such as ethanol in gasoline. As ethanol is typically blended at the distribution terminal (and not at the refinery gasoline blend header), problems arise in the operation of the overall manufacturing and distribution process. Ethanol-free gasoline is typically produced within a refinery as a finished product which fully meets all necessary specifications for sale as an ethanol-free product. This finished gasoline can be manufactured to fit the required product specifications very precisely because analytical data for the product can be obtained during the manufacture (aka gasoline blending) process and used to control the blending process. As a consequence, manufacturing costs are kept to a minimum because expensive blendstocks are usually not wasted by exceeding specifications. Unfortunately, this type of precise manufacturing control is not possible for blending configurations where the final commercial grade ethanol-containing gasolines are prepared by mixing a non-ethanol containing subgrade blend manufactured at a refinery with ethanol at a location remote from the refinery.
As explained in U.S. Pat. No. 6,258,987 (Schmidt), the ethanol is not usually blended into the finished gasoline within the refinery because the ethanol is water soluble. As a consequence of this solubility, an ethanol-containing gasoline can undergo undesirable change if it comes in contact with water during transport through a distribution system, which may include pipelines, stationary storage tanks, rail cars, tank trucks, barges, ships and the like: absorbed or dissolved water will then be present as an undesirable contaminant in the gasoline. Alternatively, water can extract ethanol from the gasoline, thereby changing the chemical composition of the gasoline and negatively affecting the specification of the gasoline, possibly leading to regulatory violations since the government may require a certain oxygenate content in the gasoline sold at the pump. Government regulation in the U.S., for example, has until recently limited the oxygen content of gasoline to 4.0 wt. % while also requiring that reformulated gasolines contain at least 1.5 wt. % of oxygen, resulting in the gasoline known as E10 when ethanol is used as the oxygenate at nominally 10 vol %. More recent regulations propose a grade known as E15 for newer vehicles and other grades are also on sale, for example, E85, for use in multi-fuel engines.
In order to avoid contact with water as much as possible, ethanol-containing gasoline is usually manufactured by a multi-step process in which the ethanol is incorporated into the product at a point which is near the end of the distribution system, e.g. at the product distribution terminal, “at the rack”. More specifically, gasoline which contains a water soluble oxygenate such as ethanol, is generally manufactured by producing an unfinished and substantially hydrocarbon precursor subgrade or blendstock usually known as a Blendstock for Oxygenate Blending (BOB) at the refinery, transporting the BOB to a product terminal in the geographic area where the finished gasoline is to be distributed, and mixing the BOB with the desired amount of alcohol at the terminal.
When a BOB is manufactured at the refinery, the properties of the BOB are measured and controlled to intermediate specifications that differ from the finished E10 gasoline in order to compensate for the effects of oxygenate which will be added after the BOB leaves the refinery. The effects of oxygenates such as ethanol and methanol are variable and can depend on the chemical composition of the BOB. For example, the addition of ethanol has a substantial effect on gasoline volatility as well as the distillation curve, and the magnitude of this effect is dependent on the chemical composition of the BOB. In addition, blending ethanol into gasoline results in a non-ideal solution that does not necessarily follow linear blending relationships.
The variable effects which result when an oxygenate such as ethanol, is mixed with a subgrade blend (BOB) to form a finished gasoline are taken into account by setting BOB manufacturing specifications that are different than the finished E10 gasoline commercial specifications to account for the Ethanol effect. These BOB specifications include a margin for error to accommodate the variable effect of the oxygenate, e.g. ethanol. In addition to the variability of the effect of the oxygenate on the intrinsic property value of the finished gasoline, additional variability can be introduced into the measured results of the finished gasoline property due to the effect of the oxygenate on the intrinsic variability of the analytical test method, and sample handling related to addition of the oxygenate in the laboratory. Because failure to adequately allow for the margin for error can lead to violation of the required commercial specifications for the finished gasoline, this can add cost to the manufacturing process since more expensive blendstocks may be required to achieve the necessary margin for error.
Various proposals have been published for avoiding the need for blending the subgrade to excessively stringent specification in order to ensure regulatory compliance when the oxygenate is added at a distant location. U.S. Pat. No. 6,258,987, mentioned above, for example, proposes a process which involves withdrawing a sample of the subgrade, mixing it with a known amount of alcohol, analyzing properties of the mixture, and using the analysis results to control and optimize the blending process.
US 2009/0292512 (Wolf), while not dealing directly with the manufacturing offset issue, does recognize that the vapor pressure of oxygenated fuels, particularly alcohols such as ethanol, propanol and butanols, and esters, ketones, etc., are non-ideal, and complicate the blend models for such oxygenated fuels and proposes a method for predicting the distillation characteristics of oxygenated blends.
US 2009/0158824 (Brown) proposes a method for analyzing gasoline or diesel fuel products and certifying their quality for regulatory purposes. In this method, a representative sample of a manufactured petroleum refinery product is analyzed and certified upon completion of product manufacture using the on-line process analyzer(s) used to monitor the manufacture of the product. The representative sample is re-introduced into the on-line process analyzer(s) for analysis, with the on-line process analyzer(s) operating strictly as a product certification analytical system. By using the same process analyzers to both monitor and also certify manufactured petroleum product, manufacture offset from specification can be reduced and the reduction in manufacturing offset directly reduces the cost of the blended or manufactured product while maintaining the same risk of non-conformance. While not addressing the problem of oxygenate blending directly, this technique would find application in certifying oxygenate-containing gasoline blends with greater certainty and reduced manufacturing costs if the reintroduced sample were to include oxygenate in the prescribed percentage proportion.
A recently published proposal in US 2010/0131247 (Carpenter) for controlling the composition of the subgrade which will yield an oxygenate-containing gasoline meeting specification when mixed with the desired amount of oxygenate involves modeling the BOB subgrade using spectroscopic measurements and associating the subgrade characteristics in the model to the properties of the finished oxygenate-containing gasoline. In this way, a laboratory analysis for the oxygenate-containing gasoline properties can be predicted and used to control and optimize the blending process for the subgrade. The use of chemometric models to predict the oxygenate-containing finished gasoline properties from spectroscopic data for the subgrade BOB enables on-line spectroscopic analysis of a product stream to make necessary adjustments to blend the components of the BOB to maintain oxygenate-containing finished gasoline properties based on model predictions.
While the use of chemometric models as described in US 2010/0131247 represents one way to assure compliance of the finished gasoline with specification, the development of the required, highly detailed models is itself time-consuming and possibly subject to error arising from misinterpretation and correlation between the properties of the finished gasoline and those of the BOB subgrade.